As an optical sensor with high precision, for example, there is a so-called optical heterodyne sensor (for example, refer to Non-patent Document 1) in which interference occurring when mixing signal light modulated by a measured object with a local oscillation signal having a different frequency from the signal light is received and detected in a photoelectric detector. However, due to the complicated structure, a current detection sensor that is based on a principle of modulation of light intensity using the Faraday effect and a voltage detection sensor that is based on a principle of modulation of light intensity using the Pockels effect are recently being developed for practical use.
The basic operation of the optical intensity modulation type sensors is as follows.
a) The intensity of light transmitted from a light source to a sensor head is modulated by a measured object.
b) The light whose intensity has been modulated is transmitted to a light receiving element and is converted into an electrical signal proportional to the amount of light in the received signal.
c) A change in the amount of received light is acquired from a change in a value of the electrical signal, such that the size of the measured object is known.
In such an optical intensity modulation type sensor, it is an important issue in securing the measurement accuracy to inhibit a change in the amount of light in the received signal, which occurs due to factors other than the measured object, from affecting a sensor output.
Causes of fluctuation in the amount of light may be attributed to fluctuation in the light source intensity, a fluctuation in the transmission efficiency of a transmission line used to transmit light to a sensor head, and fluctuation in the transmission efficiency of a transmission line from the sensor head to a light receiving element. In addition, each fluctuation in the amount of light may be divided into a drift fluctuation and a relatively high-speed fluctuation.
In order to inhibit the fluctuation in the amount of light from affecting the output, the following measures disclosed in Non-patent Document 2 are taken in addition to hardware measures, such as ensuring robustness of an optical system.
1) Calculation of a degree of modulation: an AC component and a DC component are acquired from the received electrical signal obtained by converting the received signal light by using a filter, and a ratio (degree of modulation) of both components is calculated.
2) Stabilization of energy input to a light source: a control is made such that electrical energy input to the light source is constant.
3) Control of the light source intensity: electrical energy input to an element in order to drive a light source element is controlled such that the average time of the amount of received signal light is constant.
In principle, both measures described in 2) and 3) cannot be performed at the same time.
Non-patent Document 1: ‘Analyses of basic characteristics of photocurrent transformer applying an optical heterodyne method’, the Institute of Electrical Engineers of Japan, journal B. Vol. 117, No. 3, 1997 (pp. 354-363)
Non-patent Document 2: ‘Technical data on optical fiber sensor: revised edition’, edited by Toshihiko Yoshino, 1986 (pp. 404-405)